The present invention relates to code division multiple access (CDMA) communication in cellular radio telephone communication systems, and more particularly, to a multipath search processor employed in a CDMA RAKE type receiver.
Direct sequence code division multiple access (DS-CDMA) allows signals to overlap in both time and frequency so that CDMA signals from multiple users simultaneously operate in the same frequency band or spectrum. In principle, a source information digital data stream to be transmitted is impressed upon a much higher rate data stream generated by a pseudo-random noise (PN) code generator. This combining of a higher bit rate code signal with a lower bit rate data information stream xe2x80x9cspreadsxe2x80x9d the bandwidth of the information data stream. Accordingly, each information data stream is allocated a unique PN or spreading code (or a PN code having a unique offset in time) to produce a signal that can be separately received at a receiving station. From a received composite signal of multiple, differently coded signals, a specifically PN coded information signal is isolated and demodulated by correlating the composite signal with the specific PN spreading code associated with that specific information signal. This inverse de-spreading operation compresses the received signal to permit recovery of the original data signal and at the same time suppresses interference from other users.
In addition to receiving signals transmitted from several different transmitting information sources, a receiver may also receive multiple, distinct propagation paths of the same signal transmitted from a single transmitter source. One characteristic of such a multipath channel is an introduced time spread. For example, if an ideal pulse is transmitted over a multipath channel, the received corresponding signal appears as a stream of pulses, each pulse or path having a corresponding different time delay, as well as different amplitude and phase. Such a complex received signal is usually called the channel impulse response (CIR). Multipaths are created in a mobile radio channel by reflection of the signal from obstacles in the environment such as buildings, trees, cars, people, etc. Moreover, the mobile radio channel is dynamic in the sense it is time varying because of relative motion affecting structures that create the multipaths. For a signal transmitted over a time varying multipath channel, the received corresponding multiple paths vary in time, location, attenuation, and phase.
The existence of multiple paths, however, may be used to advantage in a CDMA system using signal diversity combining techniques. On advantage concerns signal fading which is a particular problem in mobile communications. Although each multipath signal may experience a fade, all of the multipaths usually do not fade simultaneously. Therefore, a diversity-combined signal output from a CDMA receiver is not adversely affected by a temporary fade of one multipath.
A CDMA receiver in accordance with the present invention employs a multipath search processor that searches for and identifies the strongest multipaths along with their corresponding time delays. A RAKE demodulator captures most of the received signal energy by allocating a number of parallel demodulators (called RAKE xe2x80x9cfingersxe2x80x9d) to the strongest multipath components of the received multipath signal as determined by the multipath search processor. The outputs of each of the RAKE fingers are diversity-combined after corresponding delay compensation to generate a xe2x80x9cbestxe2x80x9d demodulated signal that considerably improves the quality and reliability of communications in a CDMA cellular radio communications system.
The multipath search processor, (sometimes referred to herein as simply xe2x80x9csearcherxe2x80x9d) used in accordance with an example embodiment of the present invention identifies the channel impulse response of a complex received signal in order to extract the relative delays of various multipath components. The searcher also tracks changing propagation conditions resulting from movement of the mobile station or some other object associated with one of the multipaths to adjust the extracted delays accordingly.
More specifically, the channel impulse response of a received multipath signal is estimated within a certain range of path arrival times or path arrival delays called a search window. All signals detected within the search window form the delay profile, but only those signals originated from the transmitter belong to the channel impulse response. The remaining received signals in the delay profile are noise and interference. When the signals forming the delay profile are represented by their respective powers and delays, the delay profile is called power delay profile (PDP).
The channel impulse response is estimated very frequently so that delay variations, of the radio channel can be tracked. In particular, the position of the channel impulse response within the search window frequently changes because of movement of the mobile station or other object motion as well as from frequency mismatch of the PN sequence generators used at the transmitter for spreading and at the receiver for de-spreading. As a result, the position of the search window must be adjusted to keep the channel impulse response in the middle of the search window.
Thus, it is an object of the present invention to adapt the search window position in order to maintain accurate alignment between the estimated channel impulse response and the search window.
It is also an object of the present invention to provide a methodology for accurately and efficiently determining the center of the channel impulse response so that adjustment of the search window is made to an accurate center position.
It is a further object of the present invention to adapt each selected multipath delay in accordance with the search window adjustments.
It is also an object of the present invention to provide a methodology for efficient filtration of a search window delay adjustment signal in order to minimize the influence of noise and interference.
The present invention provides a search window delay tracking procedure for use in a multipath search processor of a CDMA radio receiver. A channel impulse response is estimated for a received signal containing plural paths, each path having a corresponding path delay. A search window defines a delay profile that contains (1) the plural multipath components of the received signal forming the channel impulse response (CIR) and (2) noise and interference signals at delays where the transmitted multipath components do not exist. A mean or average delay is calculated for the estimated channel impulse response, and an error is determined between the mean CIR delay and a desired delay position corresponding to the center of the CIR search window. An adjustment is made to reduce that error so that the center of the search window and the mean CIR delay are aligned. The error may be processed either linearly (in one embodiment) or non-linearly (in another embodiment) to minimize the error and to reduce an influence of noise.
A non-linear filtering process includes calculating a delay spread from the mean CIR delay calculated for successive iterations, each iteration corresponding to a processing cycle of the window delay tracking procedure corresponding to each new input. A difference is determined between the successive delay spreads. A search window adjustment signal is set equal to the error signal if the difference is less than or equal to a threshold. Alternatively, the adjustment signal is set to zero if the difference is greater than the threshold. Consequently, if the delay spread in the current iteration is significantly different from the delay spread in the previous iteration, the new error sample calculated in the current iteration is considered unreliable, and no adjustment is made.
In a preferred example embodiment, the present invention is applied to a radio base station that includes plural sectors, each one of the sectors having one or more directive antennas receiving a signal from a mobile station that contains multiple paths. Each path has a corresponding delay. A multipath search processor at the base station includes plural channel estimators, one corresponding to each of the plural sectors. Each channel estimator generates a delay profile within a search window containing the actual channel impulse response as well as noise and interference. A path selector in the multipath search processor selects paths with strongest signals from the delay profiles generated by each channel estimator and outputs a selected channel impulse response made up of the corresponding delay and power for each selected path. A window tracking unit maintains alignment of a center of the selected channel impulse response and a center of the search window. A demodulator demodulates the selected paths and combines the demodulated paths into a combined received signal. The window tracking unit adjusts the search windows in all channel estimators to maintain the center alignment as well as adapts the delays for the selected paths according to any search window adjustment.
The window tracking unit in particular calculates a mean delay from the selected paths, determines an error between the mean delay and the center of the search window, and adjusts the position of each search window in order to reduce the error. A window tracking unit controller calculates a delay spread for each new selected channel impulse response, determines a difference between the successive delay spreads, and sets an adjustment signal proportional to the error signal if the difference is less than or equal to a threshold, or sets the adjustment signal equal to zero if the difference is greater than the threshold.